In a recent digital telephone for automobile, there is used a method in which a portable telephone changes a transmission output level in accordance with the distance between the portable telephone and a base station in order to efficiently use an electric power of the portable telephone and to reduce a burden of the base station.
The present invention relates to an apparatus for controlling a transmission output level for a burst signal which is used in a communication of the TDMA system in which the transmission output level can be arbitrarily set.
In the case of outputting a radio wave by a transmitter, there is a situation such that a gain of an amplifier changes due to a temperature or a variation in parts and the transmission output level changes. As a circuit and a method for preventing such a situation and for keeping the transmission output level of the transmitter to a predetermined level, what is called an ALC (Automatic Level Control) has been known.
An example of a conventional ALC apparatus for a burst signal and a method of setting the transmission output level will be described hereinbelow with reference to the drawings.
FIG. 6 shows a conventional ALC apparatus.
Reference numeral 61 denotes an input terminal to which a signal of a predetermined amplitude is inputted; 62 indicates a variable gain amplifier whose gain can be changed through a control terminal 621; 63 a detecting circuit for extracting and detecting a part of a voltage which is proportional to an output electric power of the variable gain amplifier 62; 64 an output terminal from which a burst signal is outputted; and 66 a reference waveform generating apparatus for outputting an envelope for the burst signal. In the reference waveform generating apparatus 66, reference numeral 661 denotes a controller; 662 indicates a counter; 663 an ROM (Read Only Memory); 664 a D/A converter; and 665 an LPF (Low Pass Filter). Reference numeral 65 denotes a comparison amplifier for amplifying a difference voltage between a voltage (hereinafter, referred to as a detection output voltage) detected by the detecting circuit 63 and an output voltage of the reference waveform generating apparatus 66. An output of the comparison amplifier 65 is connected to the control terminal 621 of the variable gain amplifier 62. Reference numeral 651 denotes a minus (-) terminal of the comparison amplifier 65 and 652 indicates a plus (+) terminal thereof.
The function to keep the predetermined transmission output level and the function to generate the burst signal in the conventional ALC apparatus in FIG. 6 will now be described.
The function to keep the predetermined transmission output level will be first described. For simplicity of explanation, it is now assumed that an output of the reference waveform generating apparatus 66 is set to a predetermined voltage and is unchanged.
The input signal of the predetermined amplitude which was inputted to the input terminal 61 is amplified by the variable gain amplifier 62. A part of an output electric power of the variable gain amplifier 62 is extracted and detected by the detecting circuit 63. The output of the reference waveform generating apparatus 66 is preset to the predetermined voltage corresponding to the predetermined transmission output level. The comparison amplifier 65 amplifies the difference voltage between the detection output voltage which was inputted to the (-) terminal 651 and the input voltage to the (+) terminal 652 and outputs a control voltage of the variable gain amplifier 62. The above loop constructs a negative feedback loop such that when the input voltage to the (-) terminal 651 increases, the output voltage of the comparison amplifier 65 decreases and when the input voltage to the (-) terminal 651 decreases, the output voltage of the comparison amplifier 65 increases. When the transmission output level is held constant and is stable, the difference voltage is set to almost zero (ideally, zero).
In this state, assuming that the input signal level to the input terminal 61 increased due to some causes, the output level of the variable gain amplifier 62 increases, the detection output voltage rises, and the input voltage to the (-) terminal 651 of the comparison amplifier 65 increases. Since the input voltage to the (+) terminal 652 does not change, the control voltage as an output of the comparison amplifier 65 decreases. The gain of the variable gain amplifier 62 decreases and the output level changes in the decreasing direction. The transmission output level is held constant owing to the function as mentioned above. Even in the case where the input signal level to the input terminal 61 decreased, the operation opposite to that mentioned above is performed, so that the transmission output level is always held constant.
It can be said that the input voltage to the (-) terminal 651 of the comparison amplifier 65 follows the input voltage to the (+) terminal 652 due to the above function. Therefore, when the transmission output level is varied, it is sufficient to change the input voltage to the (+) terminal 652 of the comparison amplifier 65. For instance, when the input voltage to the (+) terminal 652 increases, the control voltage of the variable gain amplifier 62 rises and the gain increases. Since the input signal level to the input terminal 61 is constant, the transmission output level increases. On the contrary, when the input voltage to the (+) terminal 652 is reduced, the transmission output level decreases.
In the case of shaping the transmission output waveform of the output terminal 64 into a burst-like shape, it is sufficient to periodically change the input signal waveform to the (+) terminal 652 in accordance with an envelope of a desired burst signal. Thus, the gain of the variable gain amplifier 62 periodically changes. Since the input signal level to the input terminal 61 is constant, the output waveform changes like a burst-shape.
The varying process of the transmission output level and the waveform shaping process of the burst signal are executed by the reference waveform generating apparatus 66. In this case, the information of the transmission output level and the information of the envelope of the burst signal are preliminarily stored into the ROM 663.
When a reference waveform is outputted, in the reference waveform generating apparatus 66, a clock signal is inputted from the controller 661 to the counter 662. Each time a clock signal pulse is inputted, the counter 662 counts it and sends the count value as address information to the ROM 663. The ROM 663 outputs a corresponding digital signal from the input address information to a digital input terminal of the D/A converter 664. The D/A converter 664 converts the digital signal which was inputted to the digital signal input terminal to the analog signal and outputs to the LPF 665. The LPF 665 smoothes an output signal from the D/A converter 664 and supplies the smoothed signal as a reference waveform to the (+) terminal 652 of the comparison amplifier 65. By previously writing the burst-shaped waveform data corresponding to the transmission output level into the ROM 663, the input voltage to the (+) terminal 652 of the comparison amplifier 65 changes in accordance with the waveform data. Since the input waveform to the (-) terminal 651 of the comparison amplifier 65 also changes in accordance with the input waveform to the (+) terminal 652, the transmission output waveform is shaped to the burst-shaped waveform data corresponding to the transmission output level written in the ROM 663.
FIG. 7 shows outputs of the respective sections when the burst signal waveform is shaped. When the output changes such that 71.fwdarw.72.fwdarw.73.fwdarw.74.fwdarw.75 each time of t.sub.1, t.sub.2, t.sub.3, t.sub.4, and t.sub.5, an operating point of the control voltage to gain characteristics of the variable gain amplifier 62 changes such that 711.fwdarw.712.fwdarw.713.fwdarw.712.fwdarw.711. The transmission output waveform changes such that 76.fwdarw.77.fwdarw.78.fwdarw.79.fwdarw.710.
However, the construction as mentioned above has the following problems.
FIG. 8 is a diagram for explaining an influence by a resolution of the D/A converter. Reference numeral 84 denotes rising waveforms of two reference waveform generating apparatuses 66 having different transmission output levels (transmission output levels are set to P.sub.1 and P.sub.2 P.sub.1 &gt;P.sub.2). Reference numeral 85 indicates output waveforms of the D/A converters 664. Reference numeral 81 denotes a minimum resolution e of the D/A converter. In the reference waveform generating apparatus, since the D/A converter is used at the final stage, the respective ratios of the minimum resolution e to the transmission output levels differ. (e/P.sub.1 in the upper graph in the diagram and e/P.sub.2 in the lower graph in the diagram: e/P.sub.1 &lt;e/P.sub.2). Therefore, as the transmission output level decreases, it is difficult to execute the burst-shaped waveform shaping at a high fidelity.
On the other hand, since an ROM is generally used to store the reference waveform data, in the case of setting a plurality of transmission output levels, a large ROM capacity corresponding to such transmission output levels is needed.